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Re: Solid Grounding ???

Member

Registered: 09/15/03
Posts: 649
Loc: boston, ma


I am only familiar with the terms zero sequence and positive sequence in another context. When analyzing the effects of harmonics on three phase motors, the harmonics are variously classed as positive, negative, and zero sequence harmonics. You are looking at something else, the impedance of a supply system to various sequence currents, but I think that the basic meaning is the same, and that the below will help. In a _balanced_ three phase system, the current flowing in each of the legs is 120 degrees out of phase, but with the same magnitude and frequency. In a motor, this current would set up a rotating field. The harmonic currents in each leg have relative phase angles that are _different_ from the base (or fundamental) phase angle. For example, in a balanced three phase system, the third harmonic has a phase angle difference of 360 degrees, which is another way of saying that the third harmonic in all three legs is in phase with the third harmonic on the other legs. For fifth harmonic, the phase angle is 600 degrees (5*120), which happens to be the same as 240 degrees or 120 degrees. For the fundamental, the current that flows produces the rotating field. For third harmonic, all three third harmonic voltage components are _in phases_, and there is no voltage difference between leg terminals. The net result is that third harmonic currents cannot flow unless you have a neutral available. For fifth harmonic, current can flow, but the rotating field produced rotates in the reverse direction; in other words the phase sequence is reversed. These various states are the positive, zero, and negative sequence harmonics. It is just a very crude way of describing the relative phase relations between the three legs of the system. For your application, I believe that the situation is that any three phase _unbalanced_ current flow can be described as the sum of three different _balanced_ current flows. Each one of these mathematically derived three phase sets has the same current flowing in each 'phase', and when you vector add the phase currents from each of these balanced sets, your sums are the real unbalanced current flow. No harmonics are involved; the terms positive, zero, and negative sequence are used to describe the relative phase angles of the current flow for the (mathematically derived) three phase sets. The reason for this mathematical nicety appears that you can analyze the system in terms of its _balanced_ response to these various current flow conditions, and since _any_ unbalanced condition can be described in terms of these balanced conditions, you can then use this to figure out how the system will respond to unbalanced situations. An analogy would be to describe the position of a point in space by its X,Y,Z coordinates, then describe how a system would respond to an object at X=1, how the system would respond to an object at Y=1, and finally how the system would respond to an object at Z=1, and then be able to use this information to figure out how the system would respond to an object at any location. If I understand them correctly, then the zero sequence impedance is the impedance of the system when all the current in the conductors is in phase, as could occur in a single phase to ground fault. I found the following reference, which should prove useful. Like I said, my only which involved sequence current was harmonics in motors, so I am only guessing that the below is applicable. (These are all links into the same page.) http://xnet.rrc.mb.ca/janaj/1_2_1_symmetrical_components.htm http://xnet.rrc.mb.ca/janaj/1_0_faults_on_power_system.htm http://xnet.rrc.mb.ca/janaj/1_2_2_single_line_to_neutral_fault.htm Regards, Jonathan Edelson

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#128745  11/17/03 08:22 PM
Re: Solid Grounding ???

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2707
Loc: Anaheim, CA. USA


Here's some info in close regards to your posted items:
***Zerophasesequence Reactances and Capacitances***
The Inductive and Capacitive Reactances explained here are also known as "Positivephasesequence Reactances", and are used in the conventional balancedload flow problems of 3Ø circuits. When Earth return currents (due to faults or other causes) must be calculated, Negativephasesequence and Zerophasesequence Impedances must be determined. Negativephasesequence quanities are the same as Positivephasesequence. Precise determination of the Zerophasesequence quanities is impossible, because of the variability of the Earthreturn path(s). Methods have been developed, which give sufficient results. (§"Symmetrical Components"  Wagner, C.F., and Evans, R.D.)
Zerosequence Impedance is a function of Conductor Size, spacing, position with respect to grounded areas, electrical characteristics, and the resistivity of the Earthreturn circuit. I have some formulas, which if needed (and are simple to convert to a format which posts well in the thread) can be made available.
It seems (to me) the items referenced in the IEEE article, are more directed towards Transmission circuits than towards Premisis Wiring and circuits (circuits at the customer's end). Just curious if you were aware of that and if it (the article) actually is covering Transmission circuits.
I am not trying to say the information is of no use  only that it might not apply to very many of your projects  unless those projects happen to be long distance Transmission circuits, in which case the information fully applies! (just ignore this paragraph if you wish!...Dohhh!)
As mentioned by "Winnie" (Jonathan Edelson), these terms typically are covered when Harmonics come into play. Normal areas of application using terms such like "Zerophasesequence" and/or "Positive/Negativephasesequence" in Harmonic scenarios include Motors, Motor Feeders, SCA (Short Circuit Amperes) calcs and OCPD sizes, Harmonic Filter designs, and level of THD (Total Harmonic Distortion). Other areas include Inverters, Rectifiers, Transformers, Skin Effect calcs, GFCI / GFPE sense coil designs, and Power Factor correction.
You have posted a good topic for discussion, and I hope others with more pertant data will chime in here! Just wish I could post more info, so let me know in what type of design / application are you planning to apply considerations of "_X_ phase sequence" to, or if it's just a "General / Whaddaheckisdat" question.
Does any of this make helpful sense towards your question, or have I just posted irrelevant data?
Scott35
_________________________
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!

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#128747  11/18/03 05:56 PM
Re: Solid Grounding ???

Moderator

Registered: 04/10/02
Posts: 2527
Loc: WestSouthern InnerNortheast ...


“…a solid ground connection does not provide a Zeroimpedance neutral circuit.” I think possibly the author(s) mean is that a phasetoground fault does not provide infinite shortcircuit current because the source impedance cannot be zero—given inescapable reactance (and resistance} of the “inserted” transformer winding. I am terrible at symmetrical components, but I believe that zerosequence voltage and current quantities exist where there is neutraltoground current or “neutralshift” voltage [or deviation of the neutral terminal from the electrical center of the threephase source] present during a ground fault where there is impedance in the transformer winding and neutralto ground connection and groundpath impedance. Positivesequence voltage and current quantities exist in a (normal) balanced condition. Negativesequence voltage and current exist where there are imbalances in the typically wye “source” 3ø system that do not involve the neutral connection. This seems to be consistent with §1.4.7 with respect to obtaining the system neutral. Based on §1.4.3, in standard 142, the “overvoltage” problem seems almost inconsequential in solidlygrounded systems with, compared to a highresistancegrounded system, excessive voltage to ground is prevented by connecting a resistor neutraltoground that is of a somewhat lower value than the zerosequence capacitive reactance inherent in phasetoground insulation. The associated “charging current” is fairly small—on the order of 2 amperes/MVA of the serving transformer. It seems to me that if this condition can be suitably “damped” with addition of grounding resistance, then the system will be devoid of voltages outside the phasetophase voltage triangle. Donald Beeman illustrates and compares the variations in system grounding in his 1955 Industrial Power Systems Handbook. His material is preferable for its ‘scaling’ of industrial systems versus typically larger utility distribution systems. A couple of miscellaneous links on symmetrical components: Introduction To Symmetrical Components www.selinc.com/techpprs/6066.pdf Simple Calculator www.ece.utexas.edu/~grady/ABC012.html

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